When Should We Use Alkaline Cleaning and When Should We Use Acid Cleaning in Reverse Osmosis (RO) Systems?

Reverse osmosis units are among the most important water treatment technologies, especially in sensitive industries like pharmaceuticals, medical products, and food production.

Over time, RO membranes become clogged or fouled, which reduces their performance. This requires regular cleaning using chemical solutions either alkaline or acidic  depending on the type of contamination.

But when exactly should we use each type? This article provides a clear and simple explanation.

🧪 First: Alkaline Cleaning

✅ When is it used?

Alkaline cleaning is used when the membranes are fouled by:

Organic matter (such as oils, greases, proteins, sugars).

Bacterial growth or biofilm formation.

Unusual odors or discoloration in the permeate water.

🧴 What materials are used?

Clean water (permeate or deionized water).

Sodium hydroxide (NaOH).

A specialized alkaline membrane cleaner, which helps dissolve fats and organic matter.

The pH is adjusted between 10 and 11.

The cleaning solution temperature is maintained between 30 and 35°C.

🎯 Purpose:

To break down organic matter, dissolve fats, and eliminate bacterial contamination that blocks water flow.

⚗️ Second: Acid Cleaning

✅ When is it used?

Acid cleaning is applied when the membranes are fouled by:

Inorganic scaling such as calcium carbonate, calcium or iron sulfates.

Iron or manganese oxides (often appearing as reddish or brownish stains).

Silica (in specific cases and with great caution).

🧴 What materials are used?

Clean water.

Hydrochloric acid (HCl) or citric acid.

A specialized acid membrane cleaner for dissolving mineral deposits.

The pH is adjusted between 2 and 4.

Recommended temperature: between 25 and 30°C.

🎯 Purpose:

To dissolve mineral deposits, reduce differential pressure, and restore membrane performance.

♻️ What if there is both organic and inorganic fouling?

In such cases, a two-step cleaning process is recommended:

1. First, alkaline cleaning: to remove biofilm and organic fouling.

2. Then, acid cleaning: to dissolve the mineral scaling after the biofilm is cleared.

📋 Comparison Between Alkaline and Acid Cleaning 

Type: Alkaline Cleaning

Used for: Organic matter, bacteria, oils, fats.

Materials used: NaOH + alkaline cleaner.

pH: Between 10 and 11.

Ideal temperature: 30 to 35°C.

Type: Acid Cleaning

Used for: Inorganic scaling, calcium carbonate, iron oxides.

Materials used: HCl or citric acid + acid cleaner.

pH: Between 2 and 4.

Ideal temperature: 25 to 30°C.